Purification of lead



PURIFICATIO OF LEAb Filed Nov. 27, 1956 F/G.3 FIG. 2

Gms. Na N0 ADDED U b O O I 2 A TIME IN HOURS Patented Oct. is, 1938UNITED STATES. PATENTOFFICE PURIFICATION OF LEAD Louis Scott Deitz, Jr.,Metuchen, N. J., assignorto Western Electric Company, Incorporated, NewYork, N. Y., a corporation. of New York Application November 27, 1936,Serial No. 113,036 4 Claims.- (CI. 75-78) This invention relates to thepurification of lead, and more particularly tomethods of and apparatusfor removing such impurities as zinc, arsenic, tin and 5 alloys.

It is well known that zinc, arsenic, tin and antimony may be removedfrom molten lead by oxidizing these impuritieswith solid oxidizingagents, such as sodium nitrate, in the presence of an alkali hydroxide,such as sodium hydroxide, or other fluxes. However, if the addition of,the "oxidizing agent and the intermingling of the lead .and the reagentsare not properly controlled, the process will require along time forcompletion,

or will result in the oxidation of large quantities of the lead alongwith the impurities, or elsewill require the use of excessive amounts ofreagents.

It has been suggested heretofore that molten sodium hydroxide, or amixture thereof with sodium chloride, might be employed in contact witha bath of molten impure lead, and that sodium nitrate might be addedgradually at a constant rate to the molten flux, and thereby indirectlyto the lead. However, if the quantity of the nitrate added at any timein the processes former- -ly employed was small enough to preventoxidation of large quantities of lead near the end of the process,theprocess required a long time to reach completion. Furthermore, if theimpure lead and the purifying reagents were not brought into intimatecontact or these materials-were intermingled slowly, the process wasinefllcient because of loss of oxygen through the escape ofdecomposition products of the nitrate. In addition, if the whole leadbath was not effectively brought into the reaction by proper agitationor circulation, some of the lead was permanently oxidized instead of theimpurities which normally oxidize, more readily than the lead.

Among the objects of the present invention is the provision of simple,efiicient and rapid methods of removing impurities from lead and leadalloys, either selectively or in bulk.

Another object of the invention is the'provision of simple and efficientapparatus for prac-, ticing the purification methods embodying theinvention.

In accordance withone embodiment of the invention, lead is purified byrapidly bringing suc- 60 cessive portions of a body of molten impurelead antimony from lead and lead into intimate contact with a moltenalkali hydroxide and by adding an oxidizing agent thereto at a graduallydecreasing rate. These materials are thoroughly and rapidly brought intoreactive contact with each other by means of an 5 agitator provided witha plurality of projecting pins which pass through the floating layer ofhydroxide and oxidizer and also-through a portion of the molten lead.

The above. described and other objects and fea- 10 tures of theinventionwill be apparent from the following detailed description ofspecific'embodiments thereof and the annexed drawing, in which Fig. 1 isa vertical section of a lead purifying apparatus embodying theinvention, 15

Fig. 2 is a perspective view of an agitator i )rming a part of the leadpurifying apparatus, and

Fig. 3 is a graph showing curves plotted from data pertinent to thepractice of a process embodying the invention. 20

In accordance with the invention, impurities such as zinc, arsenic, tinand antimony may be removed from lead, either selectively or in bulk, byplacing a charge ID of the lead within a kettle II, which is heated to atemperature well abovethe melting point of lead by any suitable means(not shown), such as an oil burner. The

lead may be introduced into the kettle in the molten state or it may bemelted therein, as desired. The dross which accumulates upon the 30surface of the molten lead is skimmed off and an alkali hydroxide isintroduced upon thesurface of the molten lead and forms a layer I2 ofmolten hydroxide thereon. The amount of the hydroxide introduced isdetermined by and is 35 proportional to the quantity of a particularimpurity or impurities which it'is desired to remove from the lead.

The kettle I0 is provided with an" agitator indicated generally by thenumeral H, which comprises a horizontal circular plate l5 having acircular series of pins Hi -l6 which extend upwardly from the plate IS.The plate I5 is rigidly mounted upon. a shaft l8 supported by a bearingmember I9 and the shaft is provided 45 at its upper extremity with agear 20 meshing with a gear 2| secured'upon a shaft 22 which isconnected to any suitable source of power (not shown), such as anelectric motor. The agitator u is so positioned within the kettle lo andthe the plate 15 is below the surface of the molten lead and the pinsl6l6 extend upwardly through aportion of the molten lead and entirelythrough the layer otmolt'en hydroxide upon the surface of the lead.

After. the layer of molten hydroxide has been being removed that ispresent at that time in the bath or molten lead. The impurities in thelead are oxidized by the nitrate or other oxidizing agent and theoxidized impurities are absorbed by 'the layer of molten hydroxide andare thus removed from the metal. The agitator H keeps the nitratestirred into the layer of hydroxide and also keeps the hydroxide inmotion. It also rapidly brings particles of the molten lead intointimate contact with the hydroxide thus insuring a substantiallycomplete reaction between the iiitriate carried by the hydroxide and themolten This operation is continued until all of the oxidizing agentnecessary to remove the impurity in question has been added and has hadsuificient time to react with the lead, whereupon the agitator isstopped and the hydroxide "slag is skimmed ofi of the molten lead bymeans of a perforated shovel or other suitable means. The purified leadmay then be removed from the kettle and cast into pigs or used directlyfor any purposes for which the purified lead is intended.

If there is only one impurity, such as tin, in the lead or if there isno need to separate the slags obtained by removing several impurities orto remove certain impurities'and leave others in the lead, then thequantity of hydroxide employed and the total amount of oxidizing agentused are proportional to the total quantity oi. impurities present inthe lead. However, it may be desirable to remove only one of two or moreimpurities from the lead, or it may be advantageous-to remove thevarious impurities successively and to collect the slags produced by theremoval of each impurity separately. In this eventthe purification iseffected in steps, in each of which a sufllcient quantity of hydroxideisused and enough oxidizing agent is gradually added at a constantlyreducing rate to remove only a particular impurity. When the removal ofthis particular impurity. is completed the agitator is stopped and thehydroxide slag withdrawn. These operations may be repeated if desiredfor the removal of another irnpurity, making certain that the hydroxideand oxidizer added are employed-in the quantities requisite for theremoval of this impurity. This operation may be repeated for as manytimes as may be necessary or desirable to purify the lead to the desiredextent. Ithas been found that the usual impurities in lead, such aszinc, arsenic, tin and antimony, are removed by this process in thissequence.

Specific examples of the removal of various impurities in accordancewith the invention are as follows:

Example 1.-A ten pound lot of a lead-tin alloy containing about 3% tinmay be purified to the point where it contains only about .02% tin bycovering the molten alloy with nine-tenths of a pound of sodiumhydroxide,-l. e. three parts of sodium hydroxide per part of tin in thelead, and

parts of the agitator are so proportioned that adding sixty-six grams ofsodium nitrate thereto at a constantly decreasing rate, the amount addedat any particular time depending upon the quantity of tin present in thelead at that time. The rate of addition of the sodium nitrate and'theresults obtained are indicated in the following table: 4

At end of period NaNO Period added Percent Total Percent Sn in lbs. tintin realloy removed moved Grams 1st 15 minutes 14M 1 2.3 07 23 2nd 15minute! 12 1. 8 12 40 3rd 15 minutes. 10 1.3 17 I 60 4th 15 minutes....8 1. 0 20 73 6th 15 minuts 6H 0. 7 23 as 6th 15 minutes 5 0. 5 25 7th 15minutes 4 0. 3 27 93 8th 15 minutes..- 3 0. 1 29 97 9th 15 minutes-.- 20. 03 297- 99 10th 15 minutes 1 0. 02 2% 99. 3 g

2% hours 06 Example 2.Ten pounds of another lead alloy containing about1% antimony and .0'l% tin maybe detinned by covering the molten metalwith sodium hydroxide and adding 2 grams of sodium nitrate thereto at agradually decreasing rate over a period of about thirty minutes. At theend of this period the agitatorshould be stopped and the slag withdrawn.The lead will then contain about .9% antimony and less than .02% tin.

Example 3.-Another lead alloy containing .9%

antimony and .04% tin may be purified as follows:

Ten and a half pounds of the alloy may be melted and covered with aquantity of sodium hydroxide equivalent to at least three times theamount of the total antimony and tin therein, the agitator started andsodium nitrate added to the molten material at the following rate:

After this treatment the slag should be withdrawn and the metal ,willcontain less than .02% antlmony and less than .02% tin.

In all of these examples, the lead is purified without the oxidation ofexcessive quantities of lead along with the impurities. Also thepurification is accomplished at a much more rapid rate than was possibleby the use or methods and apparatus heretofore known and requires asmaller quantity of oxidizing agent than was required by former methods.I when large quantities of impurities, such as tin and antimony, arepresent in lead, they may be much more rapidly oxidized than mayrelatively small quantities of these materials without oxidizingexcessive quantities of the lead at the same time. Consequently, inaccordance with this invention, relatively large quantities of oxidizingagent are added during the initial stages of the purification processand the quantity of oxidizint agent is gradually decreased as theprocess pro- Thlsis ,due to the tact-that.

gresses so that at nofstage isthere an appreciable the quantity ofimpurity which is to be removed excess of oxidizing agent, but at thesame time there is present a suflicient amount of oxidizing agent toreact with the-impurities at substantially the maximum rate possible forthe quantity of impurities then present.

. The quantity of oxidizing agent to be added during any given period ofthe process may be determined by ascertaining the greatest amount ofoxidizing agent that can be added to the lead over 'a fifteen minuteperiod for a, given percentage of impurity in the alloy withoutoxidizing excessive quantities of the lead along with the impurity andwithout losing the oxidizing power of the nitrate by the escape ofexcessive amounts of oxidizing gases from the hydroxide. Thisinformation may be obtained for a series of lead alloys of differentcompositions and the results plotted to obtain graphs which may beutilized to practice the invention effectively.

For instance, in purifying the lead-tin alloy of Example 1 above, themaximum quantities of sodium nitrate that could be added in a particularapparatus to a series of lead alloys containing from about 3% tin downto substantially no tin were determined. These values also represent therate at which sodium nitrate should be added to a lead-tin alloycontaining about 3% tin to remove substantially all of the tin therefromin the most effective manner and are those found in the first column ofthe table given in Example 1. The summations of these respective valueswhen plotted as a function of time give a curve such as the onedesignated "A in the graph shown in Fig. 2 of the drawing. Inthis graphthe ordinates represent the total quantities in grams of sodium nitrateadded up' to the ends of the respective fifteen minute periods to thelead-tin alloy, while the abscissas for curve A represent time in hours.By similar computations, the rate of addition of the oxidizing agent forthe removalof other impurities, "such as zinc, arsenic and antimony,from lead and lead alloys may be obtained.

If the values from the above graph for the total amounts of sodiumnitrate added up to the ends of the respective periods are plotted as afunction of the quantity of tin remaining in the lead at the ends ofthese periods, a line such as B in the chart shown on thedrawing isobtained. Since the line B" is a straight line, it is obvious thatthequantity of nitrate added at any stage of the operation is directlyproportional to the quantity of tin then in the lead.

Theoretically, the best results will be obtained if the oxidizing agentis added to the reaction kettle at a rate that is continually decreasingthroughout the operation of the process and in largescale operations itwould be feasible to provide an automatic feeder to supply the oxidizingagent in accordance with a predetermined optimum decreasing rate.However, it has been found satisfactory in actual practice to feed tothe kettle the amount of oxidizing agent required for any given fifteenminute tially uniform rate throughout that period.

While sodium nitrate has been described as the oxidizing agent employedin the specific examples given herein, other solid oxidizing agents maybe employed with satisfactory results. ple, potassium nitrate, sodium orpotassium ni trite, sodium or potassium chlorate, sodium or potassiumperoxide, and other similar solid oxidizing agents nitrate.

The amount of alkali hydroxide employed as a may be substituted for thesodium fiux should be at least three times as great asa solid oxidizingagent at period at a substan- For examfrom the lead. Instead of thesodium hydroxide employed in the specific example disclosed, po-

tassium hydroxide may be used. Also a part of the sodium or potassiumhydroxide may be replaced by-sodium chloride, or common salt, if'desired.

The slag produced in the purification process .is preferably treated inany suitable manner to remove from the excess of alkali hydroxidetherein the products obtained by the oxidizing reaction and to revivifythe hydroxide present in chemical combination with these oxidationproducts. The alkali hydroxide thus recovered from the slag may then beused again in the purification process.

The agitator employed in practicing the processes embodying theinvention very effectively mingles the impure lead with the purifyingagents and rapidly brings fresh particles of lead into intimate contactwith these agents. It also keeps the slag broken up and agitates itthor-.

- oughly. As a result,fresh particles of lead and of the refining agentsare constantly being rapidly and intimately intermingled and theimpurities in the lead arespeedily oxidized and removed. While theagitator has been disclosed with 'the plate which supports the pinsimmersed in the leadbath with the pins extending upwardly through theslag, the plate may be inverted so that it is above the contents of thekettle and the pins extend downwardly through the slag and into theupper portion ofthe lead. The plate may also be provided with openings,if desired, to reduce the mass thereof.

What is claimed is:

1. The method. of removing oxidizable impurities from molten lead whichcomprises adding an alkali hydroxide fiuxing material, oxidizing theimpurities in the lead by adding an oxidizing agent from the group ofsodium or potassium nitrate, sodium or potassium nitrite, sodium orpotassium chlorate, in successive amounts, each successive amount beingless than the preceding amount accordingly as the impurities decreaseand each amount being substantially the maximum possible to be added atthe time of addition without effecting oxidation of the lead to'anysubstantial extent, and agitating the lead as the oxidizing agent isadded.

2. The method of removing the impurities, zinc,

arsenic, tin and antimony, or any of them,from lead, which comprisesoxidizing said impurities in the presence of an alkali hydroxide byadding a gradually decreasing rate, to molten lead containing them andagitating the molten lead, the oxidizing agent being added in successiveamounts, each amount so added being substantially the maximum possibleto be added at the time of addition without effecting oxidationof leadto any substantial extent, and the successive added amounts diminishingin constant ratio to the diminishing amounts of impurity to be removedpresent at the successive times of addition.

3. The method of removing oxidizable impurities from molten lead whichcomprises adding an alkali hydroxide to the lead, oxidizing the impurities in the lead by adding a solid alkali oxidizing compound insuccessive amounts, each successive amount being less than the precedingamount as the impurities decrease and each amount being substantiallythe maximum possible to be added at the time of addition withouteffecting oxidation of'the lead to any substantial successive amounts,each successive amount being 7 4- 2,1aa,4aa

less than the preceding amount as the impurities decrease and eachamount being substantially the 'maximum possible to be added at the timeof addition without eflecting oxidation of the lead to any substantialextent, and agitating the lead 5 during the addition of the sodiumnitrate.

v LOUIS SCOI'I DEITZ, JR.

